GB2047048A - Telephone systems - Google Patents

Abstract

A private automatic branch exchange (PABX) has a unidirectional signal path the two ends of which are connected to the exchange. The signal path has a number of connecting ports spaced along its length to which telephone sets having associated pulse- code-modulation (PCM) coding and decoding apparatus may be connected. Signalling between the PABX and the telephone set occurs in PCM form on a number of channels combined in time- division-multiples (TPM), a first group of the channels being utilised for transmission from the exchange to the telephone sets and a second group, not overlapping with the first being for transmission from the sets to the exchange. The signalling path may be installed, in a building for example, following a tortuous route with a connecting port at each point at which a telephone set may be required at some future time.

Description

SPECIFICATION Telephone systems This invention relates to telephone systems. More particularly, but not exclusively, the invention is concerned with telephone systems of the type involving a private automatic branch exchange (PABX).

In a conventional PABX system, individual telephone sets are connected by individual pairs of wires to the PABX and the move of a telephone set to a new location necessitates rewiring which involves effort seemingly out of proportion to the apparent task. One object of the present invention is to provide a more flexible system in which the location of a telephone set may readily be changed.

According to the present invention, a telephone system of the kind in which signals to and from a telephone exchange are of pulse code modulation (PCM) form with a multiplicity of channels combined in time division multiples (TDM) has a distribution arrangement comprising a unidirectional signal path the two ends of which are connected to the exchange for the transmission of multi-channel TDM PCM signals, the path having a multiplicity of connecting ports spaced along its length for the connection of telephone sets each having associated electronic apparatus for PCM coding and decoding and a first group of said channels being utilised for transmission from the exchange to the telephone sets and a second group, which does not overlap with the first group, being utilised for transmission from the telephone sets to the exchange.

Preferably the second group of channels is for lowed by a guard channel which is not used for speech or other signalling to orfrom the telephone sets.

Two distribution arrangements for telephone systems in accordance with the present invention will now be described by way of example with reference to the accompanying drawings in which Figure 1 shows diagrammatically the circuit of the first arrangement, Figure 2 shows the channel allocation of the arrangement of Figure 1, and Figure 3 shows diagrammatically the circuit of the second arrangement.

Referring to Figure 1 of the accompanying drawings, the distribution arrangement now to be described has a distribution loop 1 which comprises a coaxial or other twin conductor cable 2, a first pair of conductors 3 of a feeder cable 4 from a PABX (not shown) being connected via buffer amplifier 5 to one end of the cable 2 and the other end of the cable 2 being connected via a buffer amplifier 6 to a second pair of conductors 7 of the feeder cable 4. Although shown in the drawing as a straight line, the cable 2 follows a tortuous route round a building (not shown) to points at which a telephone may at some time be required. The cable 2 is provided with frequently occurring socket outlets 8 into which telephone sets 9 can be plugged.No electronics are associated with the outlets 8 but to avoid excessive loading the plugs 10 of the telephone sets 9 may be fitted with high impedance receive and send buffers realised in large scale integrated circuit form.

To correspond with standard practice, a speech sampling rate of 8 KHz is employed, with the 125 ZS frame period being divided into 32 time slots in a conventional channel interleaved manner. To permit bothway communication, half the available channels are allocated for transmission from PABX to the telephone sets 9 and the other half for transmission in the reverse direction. The telephone sets 9 are allocated speech timeslots as required on a dynamic basis. Although there is no inherent interference between the PABX to telephone channels, the unwanted backward propagation of telephone to PABX channels can cause difficulty. This is avoided by bunching the PABX to telephone channels together, followed by the telephone to PABX channels, and then providing an unused guard channel.The worst case of potential interference occurs with a telephone set 9 located towards the end of the cable 2 and transmitting on the last channel, interferring at the beginning of the cable 2 with the first PABX to telephone channel. To avoid interference in this worst case, and assuming both ends of the loop are adequately matched, the guard channel period must exceed twice the pulse delay along the distribution loop.

Delay measurements on inexpensive pair and coaxial cables potentially suitable for the distribution loop, suggest a typical delay of 5 to 8 nsec per metre can be expected. Given a guard period of one eight-bit channel (approximately 3.9 FS) allows a maximum highway length of some 240 metres without interference. Allowing for pulse distortion and other system imperfections, a maximum practical length might be 200 metres, although, particularly in high density accommodation, lengths in excess of 100 metres are unlikely to be necessary.

With the guard period inserted, the channel allocation is as illustrated in Figure 2. Channel 0 provides the synchronisation pattern transmission from the PABX to the telephone sets 9 for bit, channel and frame synchronisation. Channel 1 is reserved for out-of-band signalling from the PABX to the telephone sets 9 for bit, channel and frame synchronisation. Channel 1 is reserved for out-of-band signalling from the PABX to the telephone sets 9, whilst channel 16 provides the signalling in the reverse direction. Channels 2 to 15 provide 14 PABX to telephone speech channels, whilst channels 17 to 30 provide the equivalent in the reverse direction.

Channel 31 is unused and provides the necessary guard period. It is also to be noted that the total delay around the system is independent of the telephone position on the loop.

The number of telephones that can be connected to an individual distribution loop is limited primarily by traffic loading considerations and the likelihood of unacceptable blocking. However, an upper limit of 100 to 200 telephones might also be imposed by the signalling methods adopted or by security considerations. The D.C. resistance of the loop is likely to be of the order of 20 to 40Q per 200 Km. Assuming power feeding from both ends, the effective feed resistance reduces to 5 to 10Q, which should be low enough to supply at least 100 low current telephones per loop whilst retaining a 50 volt supply.

An industrial building or complex would be served by as many distribution loops as was required to satisfy the likely location of telephones and traffic demands. Since the cost of the basic loops is relatively low, then considerable overprovisioning can be provided. The distribution loops originate and terminate in junction boxes where they connect to the feeder cables from the PABX. These feeder cables can be very similar two more conventional PCM systems employing separate go and return paths.

Several distribution loops can be connected to each feeder cable to provide adequate loading. However, to simplify the control, it is convenient to associate only two distribution loops with each feeder cable, and then to pair up two such systems so that a limited traffic service can be provided to all telephones in the event of a feeder cable failure. Such an arrangement is illustrated in Figure 3. The speech and signalling channels on the distribution loops 11 to 14 are simply multiplexed together on the feeder cables 15 and 16. Pulse aligners are provided to compensate for small differential delays in the loops.

Since the guard channels can be dispensed with on the feeder cables 15 and 16, a spare channel to the PABX becomes available over those cables. This channel can be used for special telephones or for interconnecting data equipment over what can be realised as effectively a separate network. If a feeder cable 15 or 16 fails, multiplex units (in the PABX) reallocate channels on the basis of six or seven channels per loop. More complicated methods can be incorporated, but this simple procedure avoids serious modification to the signalling arrangements.

The system described in the last paragraph permits a telephone set to be plugged into any loop on the complete PABX system, and for service to be available within seconds of connection. To achieve this flexibility, it is necessary for each telephone set to store internally an identity code which can be recognised by the PABX control. This identity code cannot correspond exactly with the directory number since telephones will often share one or more directory numbers. There is no need for the subscribern to have any knowledge of these code numbers.

To locate the position of a telephone in terms of the distribution loop to which it is connected, the PABX continually transmits identity codes on a cyclic basis to which individual telephone sets respond as appropriate. An individual identification code sequence of 32 bits provides for a 4 digit binary coded decimal number plus sixteen check and special bits.

Given a design limit of 5,000 extensions, a complete scan sequence requires 160 Kbits. If the scan is to be completed in 5 seconds, a scanning rate of 32 Kbit/sec is required. Since the location of the telephone sets must be assumed to be random, this scan must be provided simultaneously on all distribution loops. The location of all telephones is stored in memory.

Of the remaining 32 Kbit/sec signalling capacity, 16 Kbit/sec are used to scan for calling conditions.

This scan occur on an individual distribution loop basis. Given that a scanning message must include the code number of the telephone, again some 32 bits are necessary. Allowing a maximum of 100 telephones per distribution loop, then a calling condition can be detected in a maximum of 1/5 sec.

When a calling condition is detected, or when it is required to complete a connection to a telephone, the PABC control allocates a free speech channel and an associated free signalling channel formed from the remaining 16 Kbitisec signalling capacity. This provides the equivalent of a full duplex, out-of-band signalling channel per active telephone of just over 1 Kbitisec.

With the system organisation described, each telephone instrument has to contain a per channel codec, complex signalling logic, and means of storing the code identity. However, using modern microelectronic components, neither the cost, nor the power consumption need be excessive. Repair problems are eased since a telephone can be unplugged and returned to a central service point.

however the replacement telephone must be coded with a number not used by already connected instruments. If a new code is chosen, a revised identity code directory number relation must be stored in the PABX control. If two telephones are connected sharing the same identity code, this is readily detected providing they are on different distribution loops. If they are connected on the same loop, the particular form of erratic behaviour should soon become evident. To avoid these problems, it is preferable to integrate the telephone coding equipment with the PABX control, since this removes the need for the manual allocation or checking of allocated codes.

To avoid abuse of a system where telephones can easily be moved and reconnected elsewhere, several precautions can be taken to minimise the effect of any consequences that might occur. First, it is possible to record the movement of every telephone and to state in directory (or code) number terms the position of each telephone at any given time, and to raise an alarm when designated telephones are moved. Second, means can be incorporated where, when a telephone is moved, a check code must be keyed in before normal operation can be resumed.

Third, it can also be arranged that, when a telephone is moved, normal service cannot be resumed until a special number is inserted via the manual board or PABX control console.

As already indicated, with two distribution loops connected to a feeder cable, one channel becomes available for other purposes. Although it is possible to substitute suitable data terminals for normal telephones and obtain connection via the PABX, it is convenient to use the spare channels exclusively for data transmission on a semi-permanent plugged-up basis to the data processing complex. Packet switching may be utilised. An alternative arrangement is to provide a fixed division of slower speed channels, and adopt an approach very similar to that used for speech distribution. For example, 12 2.4 Kbit/sec bothway channels can be provided, leaving 6.4 Kbitlsec for synchronisation and signalling channels.

Claims (15)

1. Atelephone system of the kind in which signals to and from a telephone exchange are of pulse-code-modulation form with a multiplicity of channels combined in time-division-multiples having a distribution arrangement comprising a unidirectional signal path the two ends of which are connected to the exchange for the bronomission of multi-channel time-division-multiplex pulse-codemodulation signals, the path having a multiplicity of connecting ports spaced along its length for the connection of telephone sets each having associated electronic apparatus for pulse-code-modulation coding and decoding and a first group of said channels being utilised for transmission from the exchange to the telephone sets and a second group, which does not overlap with the first group, being utilised for transmission from the telephone sets to the exchange.

2. Atelephone system according to Claim 1 in which said second group of channels is followed by a guard channel which is not used for speech or signalling to or from the telephone sets.

3. A telephone system according to Claim 1 or Claim 2 in which said connecting ports are socket outlets to which plugs of telephone sets may be fitted said plugs including send and/or receive buffers.

4. A telephone system according to Claim 3 in which said send and/or receive buffers are of integrated circuit form.

5. Atelephone system according to any preceding claim in which thirty two channels are combined in time division multiples, fourteen of said channels being used for signalling from the telephone exchange to the telephone sets and a further different fourteen of said channels being used for signalling from the telephone sets to the telephone exchange.

6. A telephone system according to Claim 5 in which said fourteen channels for signalling from the telephone exchange to the telephone sets are fourteen consecutive channels of said thirty-two channels.

7. Atelephone system according to Claim 5 or Claim 6 in which said fourteen channels for signalling from the telephone sets to the telephone exchange are fourteen consecutive channels of said thirty-two channels.

8. A telephone system according to any preceding claim in which said unidirectional signalling path comprises a plurality of distribution loops at least two of which are connected by way of a feeder cable to the exchange, respective signalling channels of each of said at least two distribution loops being multiplexed together on the feeder cable.

9. A telephone system according to Claim 8 including a plurality of sets of feeder cables, each of said sets having a respective distribution loop or a respective plurality of distribution loops connected thereto.

10. A telephone system according to Claim 8 or Claim 9 in which the guard channel on the feeder cables is used for signalling between data equipments.

11. Atelephone system according to any preceding claim in which a plurality of said connecting ports have connected thereto respective telephone sets, each telephone set having provision for storing a code identifying that telephone set.

12. Atelephone system according to Claim 11 in which the exchange transmits each identity code around the unidirectional signal path and each telephone set responds when its respective identity code is transmitted.

13. Atelephone system according to any preceding claim in which a plurality of unidirectional signalling paths are provided each of said paths having a respective multiplicity of connecting ports spaced along its length for the connection of telephone sets.

14. Atelephone system substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

15. Atelephone system substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.